Most communication systems in use today consist of a transmitting station and a receiving station connected by a channel which can be free space (wireless), fibre optic cable, coaxial cable or copper wire.
To improve performance and to optimize the use of a given type of channel, the information signal is usually translated to a higher frequency to be transmitted, sent into the channel and then translated back to a lower frequency to be received. The channel normally exhibits characteristics that degrade the transmission of the signal such as fading, attenuation, additional noise and interference from other sources or other users of the channel.
The transmitter and receiver can be made adaptable to aid in reducing the effects of detrimental channel characteristics by employing circuit components with variable characteristics.
In the transmitter, a variety of transmit power levels should be available to allow the use of increased transmit power when required, relying on the lowest possible transmit power to ensure a reliable signal at the receiver while minimizing power consumption. These additional power levels should not require significantly more power or circuit complexity to implement than a comparable single power level system, nor affect the linearity of the system as the power level is changed.
In a receiver, channel variations such as fading can be adjusted for by increasing the gain in the receiver. However, large signals that are near in frequency to the desired signal can then overload the amplifier or mixer and block all reception. A solution for this is to use a variable gain selection stage early in the receiver to allow maximum gain without overload to ensure maximum dynamic range and ensure proper reception of the desired signal. Variable gain blocks can also be used in front of the demodulator or decision block to ensure a constant input signal level which is useful for decreasing the probability of reception error.
In a transmitter, the power control and frequency translation functions are usually done separately. The power control is often done in the final power amplifier or in a preceding driving amplifier, both of which require significant design effort to maintain linearity and spectral purity of the signal over a wide range of transmit powers. If the power control is done at a lower frequency, increased design effort is required in or before the frequency translation stage, often requiring increased power consumption to maintain linearity.
In a receiver, the power or gain control and the frequency translation functions are usually done separately as well. Usually the gain control is done as close as possible to the antenna to reduce the design effort required in the subsequent circuitry. This, however, often requires significant power consumption and design complexity to maintain a high dynamic range. In addition, it is greatly preferred that the transfer curve of any automatic gain control (AGC) block be linear, which can be difficult to achieve without complex circuitry.